lt3468es5-trpbf Linear Technology Corporation, lt3468es5-trpbf Datasheet - Page 8

• Manufacturer Part Number: lt3468es5-trpbf
• Description: Lt3468-1 - Photoflash Capacitor Chargers In Thinsot?
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT3468/LT3468-1/LT3468-2
Table 1. Recommended Transformer Parameters
PARAMETER
L
L
N
V
I
R
R
8
Where: V
2 in the numerator is used to include the effect of the
voltage drop across the output diode(s).
Thus for a 320V output, N should be 322/31.5 or 10.2.
For a 300V output, choose N equal to 302/31.5 or 9.6.
The next parameter that needs to be set is the primary
inductance, L
formula:
Where: V
the transformer turns ratio. I
(LT3468-1), and 1.0 (LT3468-2).
L
that the LT3468/LT3468-1/LT3468-2 has adequate time
to respond to the flyback waveform.
All other parameters need to meet or exceed the recom-
mended limits as shown in Table 1. A particularly impor-
tant parameter is the leakage inductance, L
power switch of the LT3468/LT3468-1/LT3468-2 turns
SAT
PRI
LEAK
ISO
PRI
SEC
PRI
L
PRI
needs to be equal or larger than this value to ensure
≥
OUT
10V/DIV
OUT
V
V
SW
OUT
Figure 3. LT3468 SW Voltage Waveform
is the desired output voltage. The number
PRI
is the desired output voltage. N is
V
V
. Choose L
•
N I
IN
OUT
200 10
= 5V
NAME
Primary Inductance
Primary Leakage Inductance
Secondary: Primary Turns Ratio
Secondary to Primary Isolation Voltage
Primary Saturation Current
Primary Winding Resistance
Secondary Winding Resistance
•
U
= 320V
PK
•
U
PRI
−
100ns/DIV
9
according to the following
PK
is 1.4 (LT3468), 0.7
W
LEAK
3468 G18
. When the
U
off, the leakage inductance on the primary of the trans-
former causes a voltage spike to occur on the SW pin. The
height of this spike must not exceed 40V, even though the
absolute maximum rating of the SW Pin is 50V. The 50V
absolute maximum rating is a DC blocking voltage speci-
fication, which assumes that the current in the power NPN
is zero. Figure 3 shows the SW voltage waveform for the
circuit of Figure 6(LT3468). Note that the absolute maxi-
mum rating of the SW pin is not exceeded. Make sure to
check the SW voltage waveform with V
output voltage, as this is the worst case condition for SW
voltage. Figure 4 shows the various limits on the SW
voltage during switch turn off.
It is important not to minimize the leakage inductance to
a very low level. Although this would result in a very low
leakage spike on the SW pin, the parasitic capacitance of
the transformer would become large. This will adversely
effect the charge time of the photoflash circuit.
Linear Technology has worked with several leading mag-
netic component manufacturers to produce pre-designed
flyback transformers for use with the LT3468/LT3468-1/
LT3468-2. Table 2 shows the details of several of these
transformers.
TYPICAL RANGE
Figure 4. New Transformer Design Check (Not to Scale).
100 to 300
LT3468
V
8 to 12
SW
>500
<300
0V
>1.6
<40
>5
“A”
TYPICAL RANGE
200 to 500
“B”
LT3468-1
8 to 12
>500
<500
>0.8
>10
<80
TYPICAL RANGE
3420 F07
200 to 500
LT3468-2
OUT
8 to 12
>500
<400
MUST BE
LESS THAN 50V
MUST BE
LESS THAN 40V
>1.0
<60
>7
near the target
346812fa
UNITS
mΩ
µH
nH
Ω
V
A